The invention relates to a glass composition which comprises silicon dioxide, boron oxide, aluminum oxide, calcium oxide, barium oxide and at least one alkali metal oxide.
The invention further relates to a tube manufactured from such a glass composition.
The invention also relates to a lamp envelope manufactured from the above-mentitoned glass composition.
The invention also relates to a fluorescent lamp comprising a vacuum-tight sealed glass lamp envelope in which two electrodes connected to current supply wires are provided between which a discharge takes place during operation of the lamp, the lamp envelope being filled with a gas atmosphere and a metal which during operation of the lamp at least partly evaporates, a layer comprising at least one phosphor being provided on the inside of the wall of the lamp envelope. The invention relates more particularly to a fluorescent lamp in which the metal which during operation of the lamp at least partly evaporates is mercury.
In German published Patent Application No. DE 3046594, a glass composition is described which is suitable for use in a lamp which is filled with a gas atmosphere and a metal which during operation of the lamp at least partly evaporates, which glass composition comprises SiO.sub.2, B.sub.2 O.sub.3, Al.sub.2 O.sub.3, CaO and BaO and which glass composition may in addition comprise ZrO.sub.2, MgO and SrO. The glass composition moreover contain 2-8 mol % Na.sub.2 O. It has been found experimentally that the Na.sub.2 O may not be replaced entirely or partly by Li.sub.2 O because phase separation may occur in the glass during the processing thereof. When the Na.sub.2 O is replaced fully or partly by K.sub.2 O, crystallisation occurs in the glass. The presence of at least one alkali metal oxide is desired to produce a glass having a good processibility.
During the maufacture of a lamp as described above, for example, during sealing and further shaping, the temperature of the glass of the lamp envelope is strongly increased locally, for example to above 600.degree. C. When such a lamp has a phosphor layer on the inside of the wall of the lamp envelope, a contamination of the phosphor occurs with sodium from the glass of the lamp envelope. As a result of this, the initial luminous efficiency of the phosphor is reduced. The luminous efficiency of the phosphor during operation of the lamp also decreases further in the course of time.